Hi,
Look at the table on the datasheet. It'll tell you that when '1' is pressed you'll get a connection between pins 2 and 3. When '2' is pressed you'll get a connection between pins 2 and 1 and when '3' is pressed you'll get a connection between pins 2 and 5.
This means that since '1' and '2' and '3' are on the same row the common connection to that row must be pin 2. The connection to column 1 would be pin 3, column 2 would be pin 1 and column 3 would be pin 5.
If you draw that out on paper you'll get something looking like this:
Hope it helps.
/Henrik.
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